Prince John's behavior in these scenes is, at best, underhanded and, at worst, tremendously dishonorable. He effectively lies to the rebels, telling Mowbray, Hastings and the Archbishop that he will concede to their demands, and then he reneges on his promise as soon as they have trustingly sent away their troops. The technicality that he uses to justify his action--the fact that he promised to address the rebels' complaints, not to ensure their safety--seems morally questionable. Prince John seems to go out of his way to convince the rebels that he means them no harm, repeatedly saying things like "Let's drink together friendly and embrace / That all their eyes may bear the tokens home / Of our restored love and amity" (63-65). That Hastings, Mowbray, and the Archbishop would have taken this as a promise of forgiveness seems obvious.
Prince John comes across as a much more treacherous character than any of the rebels over whom he claims moral authority. However, if we begin by assuming, as many during the Middle Ages did, that the king and the royal family are always right and have the authority of God himself behind them, then anyone who rises against them is clearly in the wrong. The royal family, thus, has the right to defeat them by any means necessary.
This line of thinking is related to the idea of the "divine right" of kings. It is an idea with obvious political value for rulers and one that was popular in the Middle Ages; the Renaissance was just beginning to question this assumption. It is obvious that at least some of King Henry's followers subscribe to this idea. When the Archbishop challenges Prince John's duplicity by asking, "Is this proceeding just and honorable?" Westmoreland replies by asking, "Is your assembly so?" This is the only answer that either he or John makes to the rebels' accusations that Prince John has broken his oath. Answering the questions only with another question, Westmoreland implies that Prince John's behavior is not wrong because it has corrected a previous wrong (i.e., "two wrongs make a right").
This concept of honor may be good enough for Prince John, and it may have been what some of Shakespeare's audience--including his ruler, Queen Elizabeth--wanted to hear. Shakespeare, however, seems to have been ambivalent about it; he has Falstaff voice his reservations about Prince John's behavior in his closing speech in IV.iii. In typical Falstaff style, he goes off into a very long, complex, and witty speech about a seemingly trivial topic--this time, wine--and expands it into a discussion of abstract truths that apply to the situation at hand.
In praising the virtue wine has in making men witty, Falstaff brings forth the virtues of a value system different from that of the king and his followers. He criticizes Prince John, in a somewhat worried tone, wishing that Prince John had "wit," for it would be "better than your dukedom. Good faith," he goes on, "this same sober-blooded boy doth not love me, not a man cannot make him laugh... There's never none of these demure boys come to any proof... They are generally fools and cowards" (84-93). Falstaff humorously blames Prince John's defects on his refusal to drink wine, but he also makes a valid criticism of Prince John's frightening lack of a sense of humor and strange version of "honor," which seems to be utterly lacking in human compassion. Falstaff knows, too, where Prince John got these bad qualities: from the leader of the state himself, King Henry IV. Even Prince Hal, he adds, is only valiant because "the cold blood he inherited of his father he hath... tilled, with excellent endeavor of drinking" (114-119).